Electronic Paint Structure with Thermal Addressing Layer

ABSTRACT

An electronic paint for an electrophoretic display includes a lower conductive layer, a thermal addressing layer disposed on the lower conductive layer, a layer of electrophoretic ink disposed on the thermal addressing layer, and an upper conductive layer disposed on the electrophoretic ink. Activation of the electrophoretic ink is based on thermal absorption of thermal radiation in a portion of the thermal addressing layer and a bias voltage applied between the upper conductive layer and the lower conductive layer.

This invention relates generally to electrophoretic displays, and morespecifically to an electronic paint including electrophoretic ink withthermal activation.

Electrophoretic display media are being developed for large displayssuch as whiteboards, signage, billboards and wall displays wheresemi-permanent images are required. Electrophoretic display media,generally characterized by the movement of particles in an appliedelectric field, can be bi-stable with display elements having first andsecond display states that differ in at least one optical property suchas lightness or darkness of a color. In recently developedelectrophoretic displays the display states occur aftermicroencapsulated particles in the electronic ink have been driven toone state or another by an electronic pulse of a finite duration, andthe driven state persists after the activation voltage has been removed.Such displays can have attributes of good brightness and contrast,wide-viewing angles, state stability for two or more states, and lowpower consumption when compared with liquid crystal displays (LCDs). Anexemplary electrophoretic display with microcapsules containing either acellulosic or gel-like internal phase and a liquid phase, or containingtwo or more immiscible fluids is described in “Process for Creating anEncapsulated Electrophoretic Display,” Albert et al., U.S. Pat. No.6,067,185 issued May 23, 2000 and “Multi-Color Electrophoretic Displaysand Materials for Making the Same,” Albert et al., U.S. Pat. No.6,017,584 issued Jan. 25, 2000.

Electrophoretic displays are often designed with various layers ofelectrophoretic and protective materials. An electrophoretic displayhaving a protective electrode is described in “Protective Electrodes forElectrophoretic Displays,” Drzaic et al., International PatentApplication No. WO0038001 published Jun. 29, 2000. The protectiveelectrode can be a vapor permeable electrode that is a reticulatedelectrically conductive structure, such as a metal screen or wire mesh,or a reticulated structure coated or impregnated with a conductivematerial.

Most currently available electrophoretic displays receive data and areaddressed by driving an active matrix, which may be located on thefrontside or backside of the display. An example of a rear-addressingdisplay is taught in “Printable Electrode Structures for Displays,”Comiskey et al., U.S. Pat. No. 6,177,921 issued Jan. 23, 2001. Oneembodiment of the display combines display materials with silicontransistor addressing structures. Active-matrix driving, however, is notan attractive option for inexpensive billboard-like displays, whichrequire only a low to extremely low refresh rate. Electronic-ink systemshave been proposed for large electrophoretic displays that have nointrinsic addressing schemes such as fixed coordinates on apixel-by-pixel grid to accurately write text and graphics. Researchersare also working on applying this digital or electronic-ink technologyto a large electronic wall display of a so-called electronic wallpaper,poster or wall screen, which could consist of a thin electrophoreticfilm placed on a wall.

An electrophoretic display that is addressable using an external stylusdevice is described in “Tiled Displays,” Albert et al., U.S. Pat. No.6,252,564 granted Jun. 26, 2001. A process for creating anelectronically addressable display includes multiple printingoperations, similar to a multi-color process in conventionalscreen-printing. The system includes one or more antennae, passivecharging circuitry, an active control system, a display, and an energystorage unit.

A paper-like medium that also employs electrophoretic particlespreferably non-fluid at room temperature and fluidic at highertemperatures is described in “Image Recording Medium, ImageRecording/Erasing Device, and Image Recording Method,” Masato et al,International Patent Application WO0043835 published Dec. 12, 2001.

A method for addressing an electrophoretic display with aphotoconductive layer is proposed in “Electrophoretic Displays inPortable Devices and Systems for Addressing such Displays,” Zehner etal., U.S. Patent Application No. 2003/0011868 published Jan. 16, 2003.Where the photoconductive layer is struck by light from thelight-emitting layer of the display, the impedance of thephotoconductive layer is lowered and the electrophoretic layer may beaddressed by an applied electric field to write an image.

While smaller electrophoretic displays often receive data and areaddressed by driving an active matrix of the display, largeelectrophoretic displays may have no intrinsic addressing schemes toaccurately write text and graphics. Various methods, systems and relateddevices have been proposed for externally addressing electrophoreticdisplays, yet their slow addressing speeds continue to be a challenge.The relatively slow switching speeds of many electrophoretic displaysresult in an external addressing device being able to transfer imagedata to the electrophoretic display much more quickly than the time thatis necessary for the electrophoretic material to be switched to thecorrect display state. Consequently, an improved electrophoretic displaysystem would allow the electronic ink to transition to the desiredoptical state while the external addressing device is moved elsewhere orremoved from the display surface.

Therefore, what is needed is a system and process whereby the effectiveaddressing time for an externally addressed electrophoretic surface isincreased, and the electrophoretic display can continue to switch fromone display state to another after the external addressing device hasmoved from one area of the electrophoretic surface to another in theprocess of transferring image data. More particularly, an improvedaddressing scheme for a larger display would allow rapid strokes of ahandheld activation device over the display surface while accommodatingthe relatively slow transition times of electronic inks. Thus, thedisplay could receive data from a handheld writing device in a shortperiod of time while allowing the electronic paint or ink to switch itsdisplay state more slowly. Such a desirable system would be costeffective for large area applications where data is updatedinfrequently, and its associated methods would be time effective.

One form of the present invention is an electronic paint for anelectrophoretic display. The electronic paint includes a lowerconductive layer, a thermal addressing layer disposed on the lowerconductive layer, a layer of electrophoretic ink disposed on the thermaladdressing layer, and an upper conductive layer disposed on theelectrophoretic ink. Activation of the electrophoretic ink is based onthermal absorption of thermal radiation in a portion of the thermaladdressing layer and a bias voltage applied between the upper conductivelayer and the lower conductive layer.

Another form of the present invention is a method of activating anelectronic paint. A bias voltage is applied between an upper conductivelayer and a lower conductive layer of the electronic paint. Thermalradiation is received on a portion of a thermal addressing layer. Atleast a portion of the received thermal radiation is absorbed in theportion of the thermal addressing layer, and electrophoretic ink isactivated based on the absorbed thermal radiation and the applied biasvoltage.

Another form of the present invention is an electronic paint activationsystem including an electronic brush and an electronic paint. Theelectronic brush includes a laser scanner and a position detector. Theelectronic paint includes a lower conductive layer, a thermal addressinglayer disposed on the lower conductive layer, a layer of electrophoreticink disposed on the thermal addressing layer, and an upper conductivelayer disposed on the electrophoretic ink. Activation of theelectrophoretic ink is based on thermal absorption of thermal radiationfrom the electronic brush into a portion of the thermal addressing layerand a bias voltage applied between the upper conductive layer and alower conductive layer of the electronic paint.

The aforementioned forms as well as other forms and features andadvantages of the present invention will become further apparent fromthe following detailed description of the presently preferredembodiments, read in conjunction with the accompanying drawings. Thedetailed description and drawings are merely illustrative of the presentinvention rather than limiting, the scope of the present invention beingdefined by the appended claims and equivalents thereof.

With thermal addressing layer 22, faster pulses or scanned beams oflight can be used to control the activation of electrophoretic ink 24 toa desired optical state, even if activation occurs at a slower timescale than the scanning process. The heated thermal addressing layerprovides a short-term storage effect to allow the scanned beam of lightto move elsewhere while the image continues to form into electrophoreticink 24.

Thermal addressing of electronic paint 10 allows the writing of an imageonto an electrophoretic display having electronic paint 10 with, forexample, a portable brush or handheld device that locally heat upportions of electronic paint 10 as it moves over electronic paint 10.The area where thermal addressing layer 22 is locally heated becomesmore electrically conductive. Thus, when a bias voltage is appliedacross upper conductive layer 26 and lower conductive layer 20, a largerelectric field is generated across the heated region of electrophoreticink 24 than across the surrounding cooler areas. The larger electricfield causes transitions from one optical state to another ofelectrophoretic ink 24, and while the bias voltage is applied andportions of thermal addressing layer 22 are warm, pixel segments ofelectronic paint 10 are switched to the desired optical state. Forexample, electrophoretic ink 24 may be switched from white to black asthermal radiation is applied and absorbed. In another example, aninitially black optical state is switched controllably to a gray orwhite state. In another example, a white optical state is switched to agray-scale optical state based on the amount of thermal energy absorbedin the thermal addressing layer 22 and the level of the bias voltage. Inyet another example, colored electrophoretic ink switches from one colorto another based on the bias voltage and the thermal absorption of theapplied thermal radiation. After writing and bias voltages are removed,electrophoretic displays incorporating electronic paint 10 continue tobe viewable with no additional power consumption.

Referring to FIG. 2, electronic paint 10 again includes lower conductivelayer 20, thermal addressing layer 22 disposed on lower conductive layer20, a layer of electrophoretic ink 24 disposed on thermal addressinglayer 22, and upper conductive layer 26 disposed on electrophoretic ink24. Layers in the stack may be formed sequentially where, for example,thermal addressing layer 22 is deposited or applied to lower conductivelayer 20 and electrophoretic ink 24 is then applied onto thermaladdressing layer 22, and then upper conductive layer 26 is deposited orotherwise applied to electrophoretic ink 24. For example, thermaladdressing layer 22 may be sputtered or evaporated onto lower conductivelayer 20. Alternatively, electrophoretic ink 24 and thermal addressinglayer 22 may be formed separately and laminated together, then coatedwith thin transparent electrode materials or metal to provide conductivesurfaces for electric field generation. Since no patterning or maskingis required, electronic paint 10 may be formed in other sequences withprocess steps such as rolling, screening, or depositions in any suitableorder. Sections or tiles of electronic paint 10 of various sizes may beassembled together or placed side-by-side to form electrophoreticdisplays of nearly any desired size that can be mounted, for example, onwalls or other large surfaces. Electronic paint 10 may be formed with asize, for example, of a few centimeters on a side to as large as onemeter by one meter or larger.

In an exemplary embodiment of electronic paint 10, images are viewedthrough transparent upper conductive layer 26, although otherembodiments allow backside viewing of or transmissive viewing throughelectronic paint 10. Reflected displays comprising electronic paint 10with a metallic backing are viewed from the top, as illustrated.Alternatively, electronic paint 10 may be viewed through lowerconductive layer 20, and can be thermally addressed from its backside.In configurations such as a transmissive display, lower conductive layer20 and thermal addressing layer 22 are transparent over the visiblelight range and electrophoretic ink 24 is selectively absorbent,allowing backside viewing of written images or optional backlighting ofthe display.

Image data including text, graphics, drawings or photos may be writtenonto electronic paint 10 by scanning thermal radiation from a scannedlaser beam onto a surface of electronic paint 10. In an exemplaryelectronic-paint display, incident radiation transmits through upperconductive layer 26 and electrophoretic ink 24, strikes thermaladdressing layer 22, and is absorbed into thermal addressing layer 22 tolocally heat electronic paint 10. Activation of electrophoretic ink 24is based on thermal absorption of thermal radiation 44 in a portion 32of thermal addressing layer 22 and on a bias voltage 34 applied betweenupper conductive layer 26 and lower conductive layer 20. As thermaladdressing layer 22 heats up, the voltage drop across thermal addressinglayer 22 lowers while the voltage drop across electrophoretic ink 24 israised. The increased electric field across electrophoretic ink 24 andthe elevated temperature of electrophoretic ink 24 increase the rate atwhich the ink will switch, allowing pixel segments of electronic paint10 to be written in a prescribed manner. As thermal addressing layer 22cools, electrophoretic ink 24 continues to transition to an intendeddisplay state as long as bias voltage 34 is applied. The desired opticalstate of electrophoretic ink 24 can be locked in or frozen by coolingthermal addressing layer 22, by removing bias voltage 34, or both.

Lower conductive layer 20 comprises, for example, a reflective metalsuch as aluminum, platinum or chrome, or a transparent electrodematerial such as indium tin oxide (ITO), a conductive polymer includingpolyethylenedioxythiophene (PEDOT) doped with polyphenylene sulfide(PPS), or other suitably conductive transparent material. With theirconcomitant higher thermal conductivity, metals tend to disperse heatmore rapidly and to locally spread the image unless they are thin.

Thermal addressing layer 22 comprises a material having a negativetemperature coefficient (NTC) of resistance, such as manganese oxide,nickel oxide, cobalt oxide, iron oxide, copper oxide, titanium oxide, asemiconductor material, a doped semiconductor material, or othersuitable NTC resistor material. A negative temperature coefficientmaterial has that property that the electrical resistance drops withincreasing temperature, with typical values of three to seven percentper degree Kelvin. Elevated temperature of thermal addressing layer 22results in lower resistance and higher electrical conductivity,therefore less voltage is dropped across the layer. Less voltage acrossthermal addressing layer 22 results in a higher voltage and therefore ahigher electric field across electrophoretic ink 24, causing fasterswitching in areas of elevated temperatures when compared to that ofcooler neighboring regions.

Local temperature increases within thermal addressing layer 22 may begenerated with focused thermal radiation from a suitable source. Thermalradiation 44 includes, for example, infrared radiation, visible light,ultraviolet light, or a combination thereof. Thermal radiation 44 may begenerated, for example, with a laser within a handheld electronic brush,and directed towards selected portions 32 of electronic paint 10 from ascanner coupled to the electronic brush.

Electrophoretic ink 24 comprises an electrophoretic material such asencapsulated electrophoretic particles that can be rotated byapplication of an electric field into a desired orientation. Theelectrophoretic particles orient themselves along the field lines of theapplied electric field and can be switched from one optical state toanother based on the direction and intensity of the electric field andthe time allowed to switch states.

Electrophoretic ink 24 may comprise one of several commerciallyavailable electrophoretic inks, commonly referred to as electronic inksor e-ink. The layer of electrophoretic ink 24 comprises, for example, athin electrophoretic film with millions of tiny microcapsules in whichpositively charged white particles and negatively charged blackparticles are suspended in a clear fluid. When a negative electric fieldis applied to the display, the white particles move to the top of themicrocapsule where they become visible to the user. This makes thesurface appear white at the top position or surface of the microcapsule.At the same time, the electric field pulls the black particles to thebottom of the microcapsules where they are hidden. When the process isreversed, the black particles appear at the top of the microcapsule,which makes the surface appear dark at the surface of the microcapsule.When the activation voltage is removed, a fixed image remains on thedisplay surface. Electrophoretic ink 24 may contain an array of coloredelectrophoretic materials selectively positioned above thermaladdressing layer 22 to allow the generation and display of coloredimages.

Before another image is written, the electronic ink of the displaymaterial may need to be reset to a well-defined state, such as an allwhite surface with white particles moved to the top of themicrocapsules, prior to re-addressing the ink. This can be accomplishedwith, for example, sustained application of relatively high voltagebetween upper conductive layer 26 and lower conductive layer 20 ofelectronic paint 10 forcing electrophoretic ink 24 into an initializedor reset optical state through the applied electric field, or byapplying thermal radiation to heat thermal addressing layer 22 whileapplying a relatively large bias voltage.

Upper conductive layer 26 comprises, for example, a transparentelectrode material such as indium tin oxide for topside viewingpurposes. It should be observed that upper conductive layer 26 and lowerconductive layer 20 do not need to be patterned or have any activematrix addressing capability. Upper conductive layer 26 is at leasttransparent to the wavelength of the activation laser light.

A backing layer comprising, for example, a sheet of glass or plastic,may be coupled to lower conductive layer 20 to increase the strength orprotection of the display while retaining the desired flexibility of thedisplay surface.

FIG. 3 illustrates an electronic paint activation system 50 including anelectronic brush 40 and an electronic paint 10. Electronic brush 40includes a laser scanner 42 and a position detector 46. Electronic paint10 includes a lower conductive layer 20, a thermal addressing layer 22,a layer of electrophoretic ink 24, and an upper conductive layer 26.Activation of electrophoretic ink 24 is based on thermal radiation 44from electronic brush 40 into a portion 32 of thermal addressing layer22 and a bias voltage 34 applied between upper conductive layer 26 andlower conductive layer 20 of electronic paint 10. With an applied biasvoltage 34 and incident thermal radiation 44 directed onto a portion 32of thermal addressing layer 22, one or more pixels can be written ontoelectronic paint 10 as desired. Thermal radiation 44 may be generated,for example, from a laser source within electronic brush 40 and directedby laser scanner 42 onto desired portions of electronic paint 10.Position detector 46 provides position input such as location androtation to accurately write the desired image.

Exemplary electronic paint activation system 50 includes a controller 52that is electrically coupled to electronic brush 40 and controls thermalradiation 44 from electronic brush 40 along with other initializationand writing functions. Controller 52, such as a microprocessor, amicrocontroller, a field-programmable gate array (FPGA), or otherdigital device may receive and execute microcoded instructions to writea desired image onto electronic paint 10. Controller 52 controls laserscanner 42 and the light striking thermal addressing layer 22 based on adetermined position of electronic brush 40.

Controller 52 may be wired or wirelessly connected to electronic brush40 with a suitable serial or parallel interface. For example, controller52 can be contained within a personal computer (PC), a laptop computer,or a personal digital assistant (PDA) and connected to electronic brush40 via a cable or a short-range wireless link such as Bluetooth™ or802.11 protocols. Alternatively, controller 52 is contained withinelectronic brush 40, and image data is provided to electronic brush 40and controller 52 via a memory device such as a memory stick, or anuplink from a PC, laptop computer or PDA that is optionally connected tothe communication network 54. Controller 52 may be connected to acommunications network 54 such as a local area network (LAN), awide-area network (WAN), or the Internet to receive and send informationto activate and transfer images onto electronic paint 10.

As electronic brush 40 is stroked or swept across the surface ofelectronic paint 10, thermal radiation 44 from laser scanner 42 isdirected preferentially at portions of thermal addressing layer 22 towrite the image data. Bias voltage 34 may be set to a fixed level aslaser scanner 42 thermally addresses electronic paint 10. Alternatively,bias voltage 34 maybe continuously varied as thermal radiation 44 fromlaser scanner 42 is scanned across the surface of electronic paint 10,while position detector 46 provides sensor information that allowscontroller 52 to determine the location and rotation of electronic brush40. The image data can be provided in real time as the image is writtenwith electronic brush 40, or stored within electronic brush 40 untilwritten.

In one embodiment, a backing layer such as a sheet of plastic or a sheetof glass is coupled to lower conductive layer 20, offering desirablerigidity and ruggedness, and helping to thermally insulate image pixelsand pixel segments from neighboring pixels.

FIG. 4 shows a cross-sectional view of an electronic paint with athermal addressing layer and a backing layer, in accordance with oneembodiment of the present invention. Electronic paint 10 includes alower conductive layer 20, a thermal addressing layer 22 disposed onlower conductive layer 20, a layer of electrophoretic ink 24 disposed onthermal addressing layer 22, and an upper conductive layer 26 disposedon electrophoretic ink 24. A backing layer 28 is coupled to lowerconductive layer 20. Backing layer 28 comprises, for example, a sheet ofplastic, a sheet of glass, a sheet of metal such as aluminum, copper ora metal alloy, or a ceramic substrate. Backing layer 28 may contain anarray of recessed regions 30 to thermally isolate pixel segments in thelayer of electrophoretic ink 24. As electronic paint 10 is thermallyaddressed, portions of thermal addressing layer 22 are locally heatedabove one or more recessed regions 30 and electrophoretic particleswithin electrophoretic ink 24 are switched to the desired optical stateaccordingly. Thermal isolation of pixel segments allows fasterswitching, higher contrast, and less bleeding of an image intoneighboring regions. Recessed regions 30 and the perimeter region may besized to provide a desired time constant for heating and cooling thermaladdressing layer 22 and to provide the desired latency time forswitching electrophoretic ink 24. Backing layer 28 may be glued,adhered, or otherwise attached to lower conductive layer 20 ofelectronic paint 10.

Recessed regions 30 may be configured with small, locally isolatedpoints or regions. In one example, the size of recessed regions 30 is onthe order of the pixel size for the display. In another example, thesize of recessed regions 30 is appreciably smaller than the pixel sizefor the display, such that more than one recessed region 30 isirradiated with thermal radiation from an applied laser beam to activateelectrophoretic ink 24. The array of recessed regions may be configuredto encompass, for examples: an array of magenta, yellow, and cyanelectrophoretic materials; an array of magenta, yellow, cyan and blackelectrophoretic materials; or an array of red, green and blueelectrophoretic materials for transmissive displays.

FIG. 5A, FIG. 5B, FIG. 5C, FIG. 5D and FIG. 5E are illustrations of amethod for activating an electronic paint having a thermal addressinglayer, in accordance with one embodiment of the present invention. Anelectronic paint 10 including a lower conductive layer 20, a thermaladdressing layer 22, a layer of electrophoretic ink 24, and an upperconductive layer 26 is exposed to various bias voltages 34 and focusedthermal radiation to control and switch portions of electronic paint 10.These cross-sectional views show electronic paint 10 under variouselectrical and thermal influences.

In an initial state seen in FIG. 5 a, electrophoretic particles ofelectrophoretic ink 24 are randomly oriented resulting in, for example,a gray or medium-colored background. Alternatively, electrophoretic ink24 may have a previously written image stored on it. Bias voltage 34 isset to zero or connections to an external voltage supply are simplydisconnected.

Bias voltage 34 is applied across upper conductive layer 26 and lowerconductive layer 20. In the step illustrated in FIG. 5 b, a negativebias voltage is applied. Due to the high electrical resistivity ofthermal addressing layer 22 and a small electric field acrosselectrophoretic ink 24, the electrophoretic particles in electrophoreticink 24 remain predominantly in their initial optical state.

When thermal radiation 44 impinges onto electronic paint 10 and biasvoltage 34 is applied to upper conductive layer 26 and lower conductivelayer 20, portions or all of thermal addressing layer 22 heat up. Thisresults in a lower electrical resistance in thermal addressing layer 22and a higher electric field across electrophoretic ink 24, therebycausing electrophoretic particles within electrophoretic ink 24 tore-orient into an initialized optical state such as a white state, asshown in FIG. 5 c. An incident beam of light or thermal radiation 44 isabsorbed in a portion of thermal addressing layer 22, and a moreconductive path is generated between lower conductive layer 20 andelectrophoretic ink 24. As the local conductivity is increased inthermal addressing layer 22, the electric field generated acrosselectrophoretic ink 24 increases and electrophoretic ink 24 is drivenaccordingly. Even when electronic paint 10 is no longer exposed tothermal radiation 44, the electrophoretic particles in electrophoreticink 24 continue their path towards their desired orientation.

Bias voltage 34 and incident thermal radiation 44 are then removed andelectrophoretic ink 24 remains in the initialized optical state untilwritten upon. When the bias voltage has been removed from electronicpaint 10, the electrophoretic particles in electrophoretic ink 24stabilize and are locked into a desired optical state.

As illustrated in FIG. 5 d, positive bias voltage 34 is applied acrosselectronic paint 10. Thermal radiation 44 is focused and applied toportion 32 of electronic paint 10. The incident radiation is partiallyor fully absorbed into portion 32 of thermal addressing layer 22.Additionally, some of the incident radiation may be absorbed directlyinto portion 32 of electrophoretic ink 24 and contribute to the localheating of thermal addressing layer 22. Electrophoretic particles inportion 32 of electrophoretic ink 24 switch optical states to produce,for example, a black pixel with white electrophoretic paint inneighboring areas.

When bias voltage 34 has been removed and thermal addressing layer 22has cooled, the electrophoretic particles of electrophoretic ink 24become frozen in their intended optical states, as seen in FIG. 5 e. Thepolarity of bias voltages, the color of electronic ink, the thickness ofthe various layers, and the aspect ratio for writing an individual pixelhave been chosen to be illustrative and instructive. The bias voltages,the color of electronic ink, the actual thickness of the materials, andthe pixel size may vary appreciably from that shown without departingfrom the spirit and scope of the claimed invention.

FIG. 6 shows a graphical representation of exemplary changes in biasvoltage, thermal radiation, temperature, electric field and ink colorwhen an electronic paint with a thermal addressing layer is activated,in accordance with one embodiment of the present invention. Bias voltagesignal 60 represents the bias voltage applied to the electronic paint.Thermal radiation intensity 62 represents the thermal radiation appliedto a portion or all of the electronic paint. Temperature curve 64 is thetemperature of a portion of the addressing layer exposed by the incidentthermal radiation. Electric field intensity 66 represents the electricfield across a portion of the electrophoretic ink as various biasvoltages and incident thermal radiation are applied and removed. Inkcolor curve 68 represents color or optical states of the electrophoreticink as bias voltage and incident thermal radiation are applied. Thetiming, magnitude and polarity of applied voltages, and thermal timeconstants for the materials in the electronic paint are intended to beillustrative, and may vary greatly from the representation shown.

At time t=t₀, the electronic paint is in a dormant or a previouslywritten state. The bias voltage is zero, and no thermal radiation from ascanned source is being applied. The temperature of the electronic paintis at an ambient or room temperature and there is no electric fieldacross the electrophoretic ink. The electrophoretic ink remains in itsinitial state, shown by ink color curve 68 as a mid-tone gray opticalstate.

At time t=t₁, a negative bias voltage is applied to the upper conductivelayer with respect to the lower conductive layer. The negative bias maybe on the order of, for example, −5 to −15 volts. Thermal radiation isnot applied, and the temperature of the electrophoretic ink remains atambient temperature with other portions of the electronic paint. A smallelectric field occurs across the electrophoretic ink, though little, ifany transition takes place.

At time t=t₂, incident thermal radiation is applied to a portion or allof the electronic paint. The thermal addressing layer heats up,decreasing the electrical resistance and increasing the electric fieldacross the electrophoretic ink. The color or optical state of theelectrophoretic ink changes according to the bias voltage and thetemperature of the thermal addressing layer; and according to the timeat which the bias voltage is applied and the thermal addressing layerstays at an elevated temperature. In the example shown, theelectrophoretic ink is switched from its current gray state to a whitestate. When the electrophoretic ink has reached the switched state,further changes do not occur even with continued application of biasvoltage and heating of the addressing layer.

At time t=t₃, the bias voltage and the incident thermal radiation areremoved. The electric field across the electrophoretic ink drops to zeroand the thermal addressing layer cools back to room temperature. Theelectrophoretic ink remains in its initialized, all-white state.

At time t=t₄, a positive bias voltage is applied, generating a smallelectric across the electrophoretic ink while causing little or nooptical state transitions.

At time t=t₅, a portion of the electronic paint is thermally addressedand heated up, increasing the local temperature of the thermaladdressing layer and increasing the electric field across theelectrophoretic ink. The electrophoretic ink in the vicinity of theheated thermal addressing layer switches optical state, such as to acompletely black optical state as shown. As the thermal radiation isremoved and directed elsewhere, the optical state of the electrophoreticink may continue to switch, if its predetermined state has not yet beenreached.

At time t=t₆, the thermal radiation is removed and the thermaladdressing layer cools down. The electric field across theelectrophoretic ink drops, and the optical state of the electrophoreticink may continue to transition until its predetermined state is reached.Ink color curve 68 indicates that the electrophoretic ink can continueto re-orient or “develop” after the incident light or thermal radiationis removed.

At time t=t₇, the bias voltage is set to zero or disconnected. Theelectric field across the electrophoretic ink drops to zero, and furthertransitions of the electrophoretic ink are curtailed. The color andintensity of the electrophoretic ink are locked in or frozen.

At time t=t₈, the electronic paint remains in its intended opticalstate, preserving the written image until refreshed, re-initialized, orwritten over by subsequent addressing of the electrophoretic ink.

FIG. 7 is a flow diagram of a method for activating an electronic paint.Various steps are described to initialize and activate an electronicpaint, such as the exemplary electronic paint shown in FIG. 2.

The electrophoretic ink is initialized to an initialized optical state,as seen at block 80. The electrophoretic ink may be initialized, forexample, to an all-white, an all-black optical state, or to a coloredoptical state depending on the type of electrophoretic ink and theapplied bias voltage. Initialization of the electrophoretic ink isaccomplished, for example, with application of a negative bias voltageand flooding or sweeping the electronic paint with thermal radiation toswitch the electrophoretic particles within the electrophoretic ink tothe initialized state. From this first optical state, theelectrophoretic can be adjusted in one common direction based on thedriving forces applied to the electrophoretic ink. The electronic paintmay be stored in the initialized state for an indeterminate period oftime or written upon forthwith.

To write on the electronic paint, a bias voltage is applied, as seen atblock 82. The bias voltage is applied between an upper conductive layerand a lower conductive layer of the electronic paint. The bias voltagemay be a fixed positive voltage or a fixed negative voltage.Alternatively, the bias voltage may vary in voltage level based on theimage data and the position of a scanned beam of laser light so that thedriving force on the electrophoretic ink is controlled.

Thermal radiation is received on a portion of the thermal addressinglayer, as seen at block 84. Thermal radiation is received, for example,from a laser scanner that projects and directs thermal radiation such asinfrared, visible, or ultraviolet light to locally heat portions of thethermal addressing layer. At least a portion of the received thermalradiation is absorbed in the portion of the thermal addressing layer. Aslight energy is absorbed, an electrically conductive path is generatedthrough the thermal addressing layer, between the lower conductive layerand the layer of electrophoretic ink. Other portions of the receivedthermal radiation may be absorbed into the electrophoretic ink, locallyheating the electrophoretic ink and the underlying thermal addressinglayer and further aiding in the transition to the desired optical state.

The thermal radiation may be received from a scanned beam of laser lightfrom an electronic brush. The electronic brush includes, for example, alaser scanner and one or more position detectors. The location androtation of the electronic brush is determined with detector signalsfrom the position detectors. The laser scanner is actuated to directlaser light from the electronic brush onto the electronic paint so thatan image may be transferred.

The electrophoretic ink is activated based on the absorbed thermalradiation and the applied bias voltage, as seen at block 86. Increasingthe local temperature of the thermal addressing layer lowers the voltagedrop across the thermal addressing layer and increases the voltage dropand electric field across the electrophoretic ink to switch the opticalstate of the electrophoretic ink to the desired state. A larger biasvoltage will increase the switching time of the electrophoretic ink. Anoptical state of at least a portion of the electrophoretic ink is setwhile the electrophoretic ink is activated. Until the addressing layeris cooled or the bias voltage is removed, the electrophoretic ink maycontinue to switch until the desired optical state is reached.

The bias voltage is removed and the electrophoretic ink stabilizes in apredetermined optical state with the removal of the bias voltage, asseen at block 88. Transitions of the electrophoretic ink may be abruptlyslowed or halted by removal of the bias voltage, thereby storing thewritten image, even as the thermal addressing layer is cooled.

Alternatively, the thermal addressing layer cools and theelectrophoretic ink stabilizes in a predetermined optical state based onthe cooling of the thermal addressing layer, as seen at block 90. Evenas the electronic brush or other thermal activator moves away from theheated portion of the thermal addressing layer, the heated portion ofthe thermal addressing layer may continue to switch the electrophoreticink as it cools. If the cooling is too rapid, heat may be dissipated toquickly and the electrophoretic ink incompletely switched. To assist incontrolled cooling of the thermal addressing layer, the backing layer ofthe electronic paint may include an array of recessed regions tothermally isolate pixel segments in the layer of electrophoretic ink.

In one embodiment, the brief exposure of the thermal addressing layer toincident thermal radiation rapidly and fully switches theelectrophoretic ink in the vicinity of the heated thermal radiationlayer. In another embodiment, the degree of incident thermal radiationand the cooling rate are controlled to allow the electrophoretic ink toreach an intermediate state in a controlled manner even after the sourceof the incident thermal radiation has moved away from the heated area.

To write image data to all portions of the electronic paint, the stepsfor activating one portion can be performed in series, in parallel, orsome combination thereof with the steps for activating another portionof the electronic paint so that the optical state of each portion is setat the desired level. In an electronic-paint system having an electronicbrush, for example, the image data is written onto additional portionsof the electronic paint as the electronic brush is moved across thesurface of the electronic paint or is lifted from the surface and newstrokes are started.

When the desired image has been transferred to the electronic paint, theimage may be viewed as seen at block 92. Further refreshing or writingof new images may occur as desired within, for example, minutes, hours,days, weeks or even months after transferring the first image.

While the embodiments of the invention disclosed herein are presentlyconsidered to be preferred, various changes and modifications can bemade without departing from the spirit and scope of the invention. Thescope of the invention is indicated in the appended claims, and allchanges that come within the meaning and range of equivalents areintended to be embraced therein.

1. An electronic paint for an electrophoretic display, said electronicpaint comprising: a lower conductive layer; a thermal addressing layerdisposed on the lower conductive layer; a layer of electrophoretic inkdisposed on the thermal addressing layer; and an upper conductive layerdisposed on the electrophoretic ink, wherein activation of theelectrophoretic ink is based on thermal absorption of thermal radiationin a portion of the thermal addressing layer and a bias voltage appliedbetween the upper conductive layer and the lower conductive layer. 2.The electronic paint of claim 1, wherein the lower conductive layerincludes one of a reflective metal and a transparent electrode material.3. The electronic paint of claim 1, wherein the thermal addressing layeris selected from the group consisting of manganese oxide, nickel oxide,cobalt oxide, iron oxide, copper oxide, titanium oxide, a semiconductormaterial, a doped semiconductor material, and a negative temperaturecoefficient material, the thermal absorbing layer having a negativetemperature coefficient of resistance.
 4. The electronic paint of claim1, wherein the thermal radiation includes one of infrared radiation,visible light, and ultraviolet light.
 5. The electronic paint of claim1, further comprising: a backing layer coupled to the lower conductivelayer.
 6. The electronic paint of claim 5, wherein the backing layerincludes one of a sheet of plastic or a sheet of glass.
 7. Theelectronic paint of claim 5, wherein the backing layer includes an arrayof recessed regions to thermally isolate pixel segments in the layer ofelectrophoretic ink.
 8. The electronic paint of claim 1, wherein theupper conductive layer includes a transparent electrode material.
 9. Amethod of activating an electronic paint, the method comprising:applying a bias voltage; receiving thermal radiation on a portion of athermal addressing layer; absorbing at least a portion of the receivedthermal radiation in the portion of the thermal addressing layer, andactivating an electrophoretic ink based on the absorbed thermalradiation and the applied bias voltage.
 10. The method of claim 9,wherein the bias voltage is applied between an upper conductive layerand a lower conductive layer of the electronic paint.
 11. The method ofclaim 9, wherein the received thermal radiation includes one of infraredradiation, visible light, and ultraviolet light.
 12. The method of claim9, wherein receiving thermal radiation on the portion of the thermaladdressing layer includes: receiving thermal radiation from a scannedbeam of laser light from an electronic brush.
 13. The method of claim 9,further comprising: setting an optical state of at least a portion ofthe electrophoretic ink while the electrophoretic ink is activated. 14.The method of claim 9, further comprising: removing the bias voltage;and stabilizing the electrophoretic ink in a predetermined optical stateresponsive to the removal of the bias voltage.
 15. The method of claim9, further comprising: cooling the thermal addressing layer; andstabilizing the electrophoretic ink in a predetermined optical statebased on the cooling of the thermal addressing layer.
 16. The method ofclaim 9, further comprising: initializing the electrophoretic ink to aninitialized optical state.
 17. An electronic paint activation system,comprising: an electronic brush including a laser scanner and a positiondetector; and an electronic paint including a lower conductive layer, athermal addressing layer disposed on the lower conductive layer, a layerof electrophoretic ink disposed on the thermal addressing layer, and anupper conductive layer disposed on the electrophoretic ink, whereinactivation of the electrophoretic ink is based on thermal absorption ofthermal radiation from the electronic brush into a portion of thethermal addressing layer and a bias voltage applied between the upperconductive layer and lower conductive layer of the electronic paint. 18.The electronic paint activation system of claim 17, further comprising:a backing layer coupled to the lower conductive layer.
 19. Theelectronic paint activation system of claim 17, further comprising: acontroller electrically coupled to the electronic brush, wherein thecontroller controls the thermal radiation from the electronic brush. 20.The electronic paint activation system of claim 19, wherein thecontroller is wired or wirelessly connected to the electronic brush.